The present invention was developed to fill a need for a device which efficiently converts thermal energy to electrical energy at relatively low operating temperatures and with power densities large enough for commercial applications. The present invention also operates in reverse mode to provide efficient cooling.
Thermionic energy conversion is a method of converting heat energy directly into electrical energy by thermionic emission. In this process, electrons are thermionically emitted from the surface of a metal by heating the metal and imparting sufficient energy to a portion of the electrons to overcome retarding forces at the surface of the metal in order to escape. Unlike most other conventional methods of generating electrical energy, thermionic conversion does not require either an intermediate form of energy or a working fluid, other than electrical charges, in order to change heat into electricity.
In its most elementary form, a conventional thermionic energy converter consists of one electrode connected to a heat source, a second electrode connected to a heat sink and separated from the first electrode by an intervening space, leads connecting the electrodes to the electrical load, and an enclosure. The space in the enclosure is either highly evacuated or filled with a suitable rarefied vapor, such as cesium.
The essential process in a conventional thermionic converter is as follows. The heat source supplies heat at a sufficiently high temperature to one electrode, the emitter, from which electrons are thermionically evaporated into the evacuated or rarefied-vapor-filled interelectrode space. The electrons move through this space toward the other electrode, the collector, which is kept at a low temperature near that of the heat sink. There the electrons condense and return to the hot electrode via the electrical leads and the electrical load connected between the emitter and the collector.
The flow of electrons through the electrical load is sustained by the temperature difference between the electrodes. Thus, electrical work is delivered to the load.
Thermionic energy conversion is based on the concept that a low electron work function cathode in contact with a heat source will emit electrons. These electrons are absorbed by a cold, high work function anode, and they can flow back to the cathode through an external load where they perform useful work. Practical thermionic generators are limited by the work function of available metals or other materials that are used for the cathodes. Another important limitation is the space charge effect. The presence of charged electrons in the space between the cathode and anode will create an extra potential barrier which reduces the thermionic current.
Typical conventional thermionic emitters are operated at temperatures ranging from 1400 to 2200K and collectors at temperatures ranging from 500 to 1200K. Under optimum conditions of operation, overall efficiencies of energy conversion range from 5 to 40%, electrical power densities are of the order of 1 to 100 watts/cm2, and current densities are of the order of 5 to 100 A/cm2. In general, the higher the emitter temperature, the higher the efficiency and the power and current densities with designs accounting for radiation losses. The voltage at which the power is delivered from one unit of a typical converter is 0.3 to 1.2 volts, i.e., about the same as that of an ordinary electrolytic cell. Thermionic systems with a high power rating frequently consist of many thermionic converter units connected electrically in series. Each thermionic converter unit is typically rated at 10 to 500 watts.
The high-temperature attributes of thermionic converters are advantageous for certain applications, but they are restrictive for others because the required emitter temperatures are generally beyond the practical capability of many conventional heat sources. In contrast, typical thermoelectric converters are operable at heat source temperatures ranging from 500 to 1500K. However, even under optimum conditions, overall efficiencies of thermoelectric energy converters only range from 3 to 10%, electrical power densities are normally less than a few watts/cm2, and current densities are of the order of 1 to 100 A/cm2.
From a physics standpoint, thermoelectric devices are similar to thermionic devices. In both cases a temperature gradient is placed upon a metal or semiconductor, and both cases are based upon the concept that electron motion is electricity. However, the electron motion also carries energy. A forced current transports energy for both thermionic and thermoelectric devices. The main difference between thermoelectric and thermionic devices is whether the current flow is diffusive (thermoelectric) or ballistic (thermionic). A thermionic device has a relatively high efficiency if the electrons ballistically go over and across the barrier. For a thermionic device all of the kinetic energy is carried from one electrode to the other. The motion of electrons in a thermoelectric device is quasi-equilibrium and diffusive, and can be described in terms of a Seebeck coefficient, which is an equilibrium parameter.
In structures with narrow barriers, the electrons will not travel far enough to suffer collisions as they cross the barrier. Under these circumstances, the thermionic emission theory is a more accurate representation of the current transport. The current density is given by:       j    =                  A        0            ⁢              T        2            ⁢              ⅇ                                            -              e                        ⁢                                                   ⁢            φ                                k            ⁢                                                   ⁢            T                                ,where A0 is the Richardson's constant, φ is the barrier height (electron work function), e is the electron charge, κ is Boltzmann's constant, and T is the temperature. Richardson's constant A0 is given by A0=(emκ2T2)/(2π22), where m is the effective electron mass and  is Plank's constant.
The diffusion theory is appropriate for barriers in which the barrier thickness (length) is greater than the electron mean-free-path in one dimension, while the thermionic emission theory is appropriate for barriers for which the barrier thickness (length) is less than the mean-free-path. However, if the barrier becomes very narrow, current transport by quantum-mechanical tunneling becomes more prominent.
There remains a need to provide a more satisfactory solution to converting thermal energy to electrical energy at lower temperature regimes with high efficiencies and high power densities.